Accurate and efficient method for predicting thermochemistry of polycyclic aromatic hydrocarbons - bond-centered group additivity

A self-consistent estimation method for the thermochemical properties of polycyclic aromatic hydrocarbons (PAH) is presented. This method is based on enthalpies of formation (DeltaHf(degrees), entropies (S(degrees)298, and heat capacities (C(degrees)p obtained from B3LYP/6-31G(d) calculations of the total energies and frequencies for 139 PAHs, including C(60) and C(70) fullerenes. The enthalpies of formation were calculated using an optimized set of homodesmic reactions given the available experimental DeltaHf(degrees) of PAHs. The theoretical entropies were compared with the existing experimental entropies, and some inconsistencies in the experimental data were identified. The estimation method presented here is a systematic extension of the widely employed atom-centered group additivity method originally proposed by Benson. This new method is based on bond-centered groups that define bonds linking two atom-centered groups and specify the size of the rings to which they belong. In addition, a term to describe the resonance energy is included. The thermochemical properties of PAHs up to C(70) fullerene are estimated with a mean average deviation of 2.8 kcal mol(-1) in DeltaHf(degrees), 0.7 cal K(-1) mol(-1) in S(degrees)298, and about 0.5 cal K(-1) mol(-1) in the C(degrees)p. This bond-centered group additivity method for the thermochemical properties of PAHs significantly expands both the range of systems that can be estimated and the accuracy of the estimations. The results of this work also allow us to assess the quality of available experimental data. For example, there are strong indications that the literature DeltaHf(degrees)of benzo[k]fluoranthene is about 10 kcal mol(-1) too low.     

High-energy collision-induced dissociation of oxosteroids derivatised to Girard hydrazones

Neutral oxosteroids have been derivatised with Girard T and P hydrazine reagents to give the corresponding Girard hydrazone quaternary ammonium salts. Both Girard T (GT) and Girard P (GP) hydrazones of oxosteroids give very intense [M](+) ion signals in electrospray (ES) mass spectra and fragment within the ES interface and collision cell to give characteristic fragment ions. GT and GP derivatives give informative high-energy collision-induced dissociation spectra, from which the structure of the precursor oxosteroid can be determined. Both charge-remote and charge-mediated mechanisms are responsible for the formation of the fragment ions at high collision-energy.     

Metal-thiophen derivatives. Organometallic compounds of palladium and platinum

Thienylmercury(II)chloride reacts with [Pd(PPh₃)₂Cl₂], [Pd(PPh₃)₄] and [Pt(PPh₃)₄] to afford new compounds containing a metal-2-thienyl linkage. The compound [Pd(PPh₃)₂(2-C₄H₃S)Cl] probably has trans stereochemistry.2-Bromothiophen undergoes oxidative addition with [Pd(PPh₃)₄] and [Pt(PPh₃)₄], probably via a radical mechanism. With [Pd(CO)(PPh₃)₃], a carbonyl inserted product is obtained. The bromo-metal(II) complexes have trans stereochemistry. The course of the reaction between 3-methyl-2-bromothiophen and Pd(PPh₃)₄ is more complex. Thus, there is evidence of some cis bromopalladium(II) compounds amongst the products, also there is good evidence to support the view that some isomerisation of 3-methyl-2-thienyl to 4-methyl-2-thienyl occurs during the reaction, thus giving greater molar quantities of [Pd(PPh₃)₂(4-CH₃-2-C₄H₂S)Br] than can be accounted for from any initial 4-methyl-2-bromothiophen impurity.The metallation of the thiophen ring, probably in the 4-position, with palladium(II) is described for 3-theylidene-4-methylaniline.     

The crystal structure of 1,1-bis(η5-cyclopentadienyl)-2,3,4,5-tetraphenylzirconole

The crystal and molecular structure of (η⁵-C₅H₅)₂Zr[C₄(C₆H₅)₄] has been determined by single crystal X-ray diffraction methods. The compound is isostructural with its titanium and hafnium analogues, and crystallizes in the monoclinic space group P2₁/n with unit cell parameters a = 13.790(5), b = 11.136(5), c = 18.692(7) », β = 92.82(4)°, and ?_calc = 1.34 g cm^?3 for Z = 4. Full-matrix least-squares refinement converged with a conventional R value of 0.049 for 2986 observed reflections. The metallocyclic ring is planar to within 0.05 », and the π-electron density is largely localized. The two independent ZrC(σ) bond lengths are 2.250(5) and 2.265(6) ». The ZrC(η⁵) distances range from 2.482(6) to 2.546(7) », and average 2.521(20) ». A comparison of zirconium- and hafnium-carbon bonds based on the data available shows that for M = Hf the MC bonds are shorter for all cases: C(sp), C(sp²), C(sp³), C(η⁵).     

Inorganic Chemistry in Liquid Ammonia : by David Nicholls,Elsevier Scientific Publ. Co., Amsterdam/Oxford/New York, 1979, x + 238 pp., Dfl. 110.00, $ 49.00.

Substitution of the hydrogen atoms of the cyclopentadienyl ring of (N, N-dimethylaminomethyl)cymantrene by deuterium changes the direction of orientation of metalation of this amine.     

Stoffwechselprodukte von Mikroorganismen. 177. Mitteilung. Avilamycin C

From cultures of the Avilamycin (Avilamycin A) producing organism, Streptomyces viridochromogenes, strain ETH 23575, a second antibiotic, Avilamycin C, C₆₁H₉₀Cl₂O₃₂ could be isolated in crystalline form. Both Avilamycins belong to the group of the Orthosomycins. By IR., ¹H- and ¹³C-NMR. spectroscopy and by transformation to a common derivative it could be proven that Avilamycin A is a methyl ketone, Avilamycin C the corresponding methyl carbinol.     

CNDO/2 calculations of some polyazaindenes

The theoretically estimated dipole moments of indolizine, imidazo[1,2-a]pyridine, imidazo-[1,5-a]pyridine and of pyrazolo[ 1,5-a]pyridine obtained by the CNDO/2 approximation have been compared with the experimental values. The bond angles and bond distances for these polyazaindenes have been estimated.     

Near UV quantum yields for rotenone and piperonyl butoxide

Rotenone and piperonyl butoxide (PBO) mixtures, so-called "synergized" rotenone, are invaluable in fisheries management where they are used to protect the habitat of endangered, native species and promote desirable gamefish populations. Continued use of synergized rotenone is threatened by inadequate control of persistence in surface water, especially where drinking water supplies are impacted. The photochemical kinetics of these chemicals were studied in the laboratory with a goal to better understand their fate in natural water. Disappearance quantum yields (phi) were determined in polychromatic light from fluorescent lamps emitting maximally at 350 nm. Rotenone, PBO and trifluralin, an actinometer, were irradiated as aqueous solutions at 25 or 50 microg L(-1) and the piscicides were determined by electrospray-liquid chromatography-mass spectrometry (ESI-LC-MS). In the photoreactor rotenone and PBO photodegraded with first-order half-lives of 500 and 220 min, respectively, and corresponding quantum yields of 0.00015 and 0.034. Rotenone absorbs sunlight strongly, while PRO does not. Differences in spectal overlap tended to counteract the disparities in phi and, in general, mathematical modeling indicates moderately rapid direct photolysis rates for both substances in surface water.     

Statistical treatment of proficiency testing data

 There are three stages to evaluating a laboratory's results in an interlaboratory proficiency test: establishing the correct result for the test item, determining an evaluation statistic for the particular result, and establishing an acceptable range. There are a wide variety of procedures for accomplishing these three stages and a correspondingly wide variety of statistical techniques in use. Currently in North America the largest number of laboratory proficiency test programs are in the clinical laboratory field, followed by programs for environmental laboratories that test drinking water and waste water. Proficiency testing in both of these fields is under the jurisdiction of the federal government and other regulatory and accreditation agencies. Many of the statistical procedures are specified in the regulations, to assure comparability of different programs and a fair evaluation of performance. In this article statistical procedures recommended in International Organization for Standardization Guide 43, Part 1, are discussed and compared with current practices in North America. Received: 22 April 1998 · Accepted: 12 May 1998